The present invention relates to a current controlled variable inductor used in an electronic tuning circuit, e.g. a tuner assembled in an automotive vehicle.
FIG. 1 is a side view illustrating a prior art variable inductor of this kind configured so that a U-shaped core 1 of magnetic material and an I-shaped core 2 of magnetic material are combined with each other, and a control coil 3 and a tuning coil 4 are wound on the core 1. The tuning coil 4 is wound through grooves 5 provided at the end portion of the core 1. By flowing a dc current or a low frequency current through the control coil 3, a closed magnetic path is formed in the cores 1 and 2 as indicated by a dotted line 6 to change the magnetic flux density based on a control of the current flowing therethrough thereby to control the effective permeability of the portion of the core 1 on which the tuning coil 4 is wound to vary its inductance. A magnetic path formed when the tuning coil 4 is energized is indicated by a dotted line 7. The grooves 5 are provided for principally adjusting the magnetic flux density of the control coil 3 across the tuning coil 4 from a structural point of view.
However, with such a structure configured so that each of the magnetic paths formed by the control coil 3 and the tuning coil 4 is formed as a complete closed magnetic path, the characteristics of the tuning coil 4, e.g. inductance and temperature characteristic, etc., are likely to vary depending upon how the cores 1 and 2 are in contact with each other. Namely, the mirror finished surface condition or slight variations in dimension in each portion 8 at which the cores 1 and 2 are in contact with each other affects various characteristics. Since the cores 1 and 2 are fixed by means of a bond in most cases, there is the possibility that the bond intrudes into the contact portion 8, resulting in a change in the contact condition. Further, since it is impossible to directly wind the control coil 3 or the tuning coil 4 on the core 1, it is required to fit coils which have been separately wound over the core 1. In addition, a process for bonding the cores 1 and 2 is required.
As stated above, the variable inductor of the conventional structure has strict requirement for accuracy needed when cores are assembled, particularly accuracy of contact portions. Thus, it is difficult to reduce variations in characteristics and the assembly work is troublesome.